The present invention is directed to rearview mirror systems for a vehicle and in particular to such systems having an automatic control that activates a reflective element between different reflective conditions in response to light levels experienced by the vehicle driver. The invention is especially advantageous when the reflective element has continuously variable reflectivity.
Automatic rearview mirrors that utilize a single sensor have been long known. However, proper control of an automatic mirror is a complex task and early efforts were not satisfactory. For example, U.S. Pat. No. 3,862,798 issued to Charles Hopkins, entitled AUTOMATIC REAR VIEW MIRROR ADJUSTER, discloses an automatic rearview mirror for a vehicle including a reflective layer covered by a liquid crystal material and a control for the liquid crystal assembly. The control includes a single photocell light sensor, facing rearward of the vehicle, and a comparator for applying a voltage to the mirror whenever the voltage developed by light impinging upon the photocell exceeds the fixed reference voltage of the comparator. The control thereby renders the mirror opaque whenever the light incident on the mirror exceeds a fixed threshold.
The problem with the Hopkins mirror is that it doesn't address the problem that different driving conditions require different light thresholds for rendering the mirror opaque. This is a result of the physiological response of the driver's eyes at different levels of background light. In order to provide a more physiologically correct response to various light sources, the prior art discloses manual adjustments by the driver of the light threshold at which the mirror is rendered opaque. Such manual adjustment can be annoying to the driver and can distract from the driving task. The prior art also discloses the addition of a separate photosensor, generally forward sensing, to serve as an ambient-light sensing means. Such dual sensor designs were intended to overcome the difficulties with the use of a single rear-facing photosensor in automatic mirrors by providing automatic adjustment of the light threshold at which the mirror is rendered opaque.
Many difficulties remain with mirror controls using two light sensors. The responsive characteristics of the pair of light sensors must be carefully selected so as to closely match. This matching is presently accomplished by comparative testing of sensors and is costly because, even with hand selection, sensors are not perfectly matched, and circuit adjustments are required for proper signal balance. Sensors also change their characteristics with age, which leads to a degradation of system performance. Variations in the vehicle in which the mirror is used can also have a profound effect on two-sensor mirrors. For example, certain vehicles have a band of light-absorbing tint across the top edge of the windshield in the vicinity of the forward-facing ambient light sensor. This tint band can greatly affect the response of the mirror to ambient light levels. Additionally, changes in the vehicle in which the mirror system is installed may occur, for example, resulting from opening or closing a sunroof or a roof of a convertible sport vehicle.